Dynamic Multistage Coupling of FeS₂/S Enables Ultrahigh Reversible Na–S Batteries

Abstract The fundamental challenges that coexisted around sulfur cathode energy storage systems, are the severe polysulfide dissolution and low reactivity resulting in poor reversibility and short cycle life, specifically, in inexpensive sodium ion batteries. Herein, the solution‐processed synthesis of ultra‐high intimate contacted FeS₂/S architecture is reported and evolution of the dynamic multistage coupling between the FeS₂ and S in sodium–sulfur batteries is revealed. Atomic visualization and in situ spectroscopy conclude that: Na_xFeS₂ (0 <x ≤1) effectively captures sodium polysulfides and promotes the conversion of S₈ to Na₂S₄ to Na₂S₂/Na₂S; simultaneously, the presence of Na₂S₂/Na₂S traps the continuous growth of iron grains during continuously discharging to 0.4 V, thereby boosting the reversibility and high capacity. Moreover, the density functional theory further analyses the unique coupling effect of Na₂S_x with different intermediate states of FeS₂. The electrode with unique structure and dynamic coupling exhibits outstanding cycle reversibility and extremely long life, which delivers a reversible capacity of 860 mAh g⁻¹ after 1000 cycles with no capacity decay at 0.5 A g⁻¹. Even under a practical areal capacity of 4 mAh cm⁻², it still shows pretty‐well cycling stability.